1. Field of Invention
The present invention relates to a stepping motor used in an OA machine for processing image information such as a facsimile, an ink jet printer, a laser beam printer, a copying machine or the like.
2. Background of Related Art
In a stepping motor used in such as the above OA machine, there has been demanded that a resolving power and a noiseless property should be enhanced to attain a low vibration and a low noise while aiming at reducing cost therefor that is an absolute condition.
The conventional technology disclosed in Japanese Patent Application Laid-Open No. Hei 8-298739 will be described with reference to FIG. 13. A rotor 5 composed of a pair of disc-like rotor cores 2 and 3 and a magnet 4 interposed therebetween is mounted on a rotary shaft 1. The rotary shaft 1 is supported by bearings 8 and 9 which are fixed by projected portions 6a and 7a of aluminum-made front casing 6 and rear casing 7, respectively. Stepped portions 6b and 7b are formed at confronting portions of the front and rear casings 6 and 7. An outer circumferential portion of a stator core 10 is supported by the stepped portions 6b and 7b. A stator coil 11 is wound around the stator core 10 and is connected to a drive circuit (not shown).
In this stepping motor, current is intermittently supplied from the drive circuit (not shown) to the stator core 11 to thereby intermittently rotate the rotor 5. The rotational property of this arrangement depends upon uniformity of a fine gap formed between an inner diameter of the stator core 10 and an outer diameter of the rotor 5. In the above-described arrangement, since the outer circumferential portion of the stator core 10 is retained by the casings 6 and 7, an eccentric amount of the outer fitting portions of the casings 6 and 7 in the assembled condition of the stator core 10 to the casings 6 and 7 is added to an eccentric amount of the inner diameter of the stator core 10 before the assembling work. As a result, the fine gap between the inner diameter of the stator core 10 and the outer diameter of the rotor 5 becomes an eccentric gap, sometimes resulting in non-uniformity.
For this reason, the arrangement suffers from a problem that the dimensional precision of the outer diameter to the inner diameter of the stator core 10 and the dimensional precision of the outer circumferential fitting portions of the casings to the dimension of the fitting portions of the bearings 8 and 9 must be high in machining.
In order to solve this problem, the arrangement is modified in design in a stepping motor as shown in FIG. 14. In this stepping motor, the stepped portions 6c and 7c are formed on the outer circumferential side of the projected portions 6a and 7a of the front and rear casings 6 and 7. The inner circumferential portion of the stator core 10 is received by the stepped portions 6c and 7c. If such an arrangement is adopted, the inner circumferential portion of the stator core 10 is directly supported. It is therefore possible to remove the eccentric factor to be generated between the inner diameter of the stator core 10 and the outer diameter of the rotor 5. Thus, it is possible to solve the above-noted problem.
FIG. 15 shows a design for the purpose of reducing the manufacture cost. The shape of the design is simplified in which, after the front casing 6 has been cast in aluminum, the front casing 6 is machined. The front casing 6 is shaped by metal-plate pressing. With this structure, it is possible to reduce the cost of the casings 6 and 7 per se but the eccentric factor to be generated between the inner diameter of the stator core 10 and the outer diameter of the rotor 5 is the same as that shown in FIG. 13. Incidentally, though the structure shown in FIG. 15 exemplifies the structure in which the plate 12 is arranged in front of the front casing 6, the plate 12 may not be used.
FIG. 16 shows a design for the purpose of enhancing the rotational gap precision of the design shown in FIG. 15. Bearing fitting portions of the metal plates are extended and the stepped portions 6c and 7c are formed in the extended portions. The inner circumferential portion of the stator core 10 is received by the stepped portions 6c and 7c. However, this structure suffers from a defect that, as shown by the arrows in FIG. 16, the magnetic field generated in the stator core 10 leaks to the casings 6 and 7 of the metal plates to considerably deteriorate the motor performance. Accordingly, in order to obviate the magnetic leakage in the structure in which the casings 6 and 7 are formed by pressing the metal plate, it is necessary to magnetically shield the inner diameter fitting portions of the stator core 10. It is impossible to take a structure in which the stator core 10 is held in the inner diameter portion.